As is well known, polyblends of rubbers with styrene/acrylonitrile type interpolymers have advantages in providing compositions of desirable properties including toughness and chemical resistance, as well as providing good formability. Generally, increasing the rubber content is advantageous in increasing the toughness, but some reduction in other properties such as gloss is generally experienced with increase in rubber content.
Prior art processes for preparing such polyblends are known wherein the rubber is grafted in emulsion with said monomers. Such prior art processes have not produced homogeneous polyblends with optimum toughness and color. This is particularly true in polymerizing monomer formulations having greater than 24% by weight of nitrile monomers, e.g. styrene and acrylonitrile will yield interpolymers of various compositions; a mixture containing a 76 % by weight styrene and 24% by weight acrylonitrile will form interpolymers having the same composition as the monomers at any conversion. However, if the acrylonitrile ranges from 30 to 70% then the interpolymer of the polyblend formed at higher conversion, i.e., greater than 85%, is not homogeneous having poor properties and color. By definition, those polyblends having greater than 30% of nitrile monomers in the interpolymers are considered high nitrile interpolymers and form high nitrile ABS polyblends.
It has not been found that the properties of the matrix and graft interpolymer phases of the polyblend becomes critical in preparing such polyblends having improved toughness and color. Monovinylidene aromatic monomers, e.g. styrene, and ethylene unsaturated nitrile monomers, e.g. acrylonitrile, have been found to copolymerize at different rates with the styrene type monomer adding to the interpolymer at faster rates. In high nitrile monomer formulations of styrene and acrylonitrile, the nitrile component increases percentagewise in the residual monomers as the conversion increases and adds as blocks of acrylonitrile to the matrix interpolymer or graft interpolymer. It has been found that these blocks of acrylonitrile lead to localized incompatibility in the polyblend reducing toughness and producing colored polyblends.
It is the objective of the present invention to provide an improved polymerization process that insures the production of ABS polyblends having homogeneous matrix and graft interpolymers giving polyblends of improved toughness and color.